The effect known as lasing results from the generally simultaneous decay of excited particles in a medium. This simultaneous decay may be initially triggered by a noise generated decay of a few particles which is augmented or reinforced by amplification of the radiation as it travels through the medium. The amplification can reach large magnitude and thereby produce high intensity laser radiation if the optical path through the medium is long enough, and the corresponding gain very high. The gain associated with a uniformly excited medium is typically a constant factor per unit length up to a limiting factor. If reflective or partially reflective elements are associated with the excited medium, the self lasing radiation may be reflected through the same medium many times, being amplified on each pass, to achieve the same effect as a substantially greater length of the medium. In either case, the self lasing intensity increases with the amplification by the medium. When the intensity is sufficiently high, the traveling radiation will significantly depopulate the excited states of the particles in the medium through which it passes. At this point, gain is also limited by the number of available excited particles.
In the application of laser excitation to uranium enrichment (as shown in U.S. Pat. No. 3,772,519 and U.S. patent application Ser. No. 328,954, filed Feb. 2, 1973, incorporated herein by reference), as in many other applications, the loss of excited particles in the generation or amplification of self lasing radiation may significantly reduce the efficiency of the system. In addition, the self lasing radiation may have an intensity sufficient to be harmful in other ways to the system for laser excitation.